A liquid-crystal display device is widely used as a monitor of a flat-screen TV set and a personal computer. The liquid-crystal display device is provided with a liquid-crystal panel, a cold-cathode tube arranged as a backlight for the liquid-crystal panel, and a lighting device for driving the cold-cathode tube.
Conventionally, the lighting device for the cold-cathode tube is formed of an inverter for supplying driving voltage to the cold-cathode tube, a dimmer circuit for adjusting luminance intensity of the cold-cathode tube, a current detector for detecting a load condition of the cold-cathode tube, a protective circuit for stopping supply of the driving voltage when the cold-cathode tube becomes a no-load condition, a control circuit for controlling an output of the inverter so that the luminance of the cold-cathode tube is maintained at a level adjusted by the dimmer circuit, and so on (Japanese Laid-open Patent Publication No. 2007-115568).
When the current detector detects that an amount of current flowing through the cold-cathode tube becomes smaller than a preset value, the protective circuit halts the operation of the inverter by assuming that a lead wire of the cold-cathode tube is broken or connectors are poorly fitted.
The inverter is configured of a piezoelectric transformer, a voltage-controlled oscillator circuit, an oscillation frequency control circuit, a driving circuit for driving the piezoelectric transformer, and so on (Japanese Laid-open Patent Publication No. 2000-243586).
In a liquid-crystal display device used in offices, a luminance adjustment function is provided to thereby usually adjust the luminance level of 50% to 100% of the maximum luminance. In this way, the minimum adjustable luminance is set at about 50% of the maximum luminance.
However, in special-purpose applications, for example, a liquid-crystal display device for use in a marine radar, it is necessary to reduce the luminance considerably so that the liquid-crystal display device does not disturb the view for night navigation. For this purpose, it is necessary to arrange the luminance adjustment function to cover a wide range of adjustment such as 0.1% to 100% of the maximum luminance. In such a case, the minimum luminance will be set at an exceptionally low level of 0.1%.
Then, the current flowing through the cold-cathode tube is drastically reduced, which may activate the protective circuit so that supply of the driving voltage to the cold-cathode tube is halted. It is possible to detect, to a certain extent, a current when it is made smaller by lowering the setting value of the current detector. However, when the luminance is adjusted at an exceptionally low level such as 1% or 0.1% of the maximum luminance, the current flowing through the cold-cathode tube becomes very small. As a result, it becomes impossible to lower the setting value to such an extent, making it difficult for the current detector to detect the current accurately and for the protective circuit to function normally.
If the protective circuit is turned off or removed in order to prevent the protective circuit from behaving unsteadily, the protective circuit never functions even if the lead wire of a cold-cathode tube is broken or connectors are poorly fitted due to transportation or handling of the liquid-crystal display device. This causes the display device to display in low luminance or with flicker. Furthermore, using the display device in such a condition may lead to a more serious malfunction. In addition, such a condition may cause unsatisfactory detection of a broken lead wire or poor connection of the connectors when the products are inspected, which may eventually cause defective products by overlooking.